1. Field of the Invention
The present invention relates to a power steering apparatus, and particularly, to a power steering apparatus which drives an oil pump by a motor to generate hydraulic pressure and assists in driver's steering through the generated hydraulic pressure.
2. Description of the Related Art
Conventionally, in a power steering apparatus which drives an oil pump by a motor to generate hydraulic pressure and assists in driver's steering through the generated hydraulic pressure, the technique of stopping a power steering function regardless of the voltage of a power source to be applied to a motor in a case where the rotating speed of the motor is equal to or less than a predetermined value is disclosed (for example, refer to Japanese Patent No. 3652142 (particularly, the description related to Claim 3)).
According to the conventional power steering apparatus disclosed in Japanese Patent No. 3652142, a fail safe function can be exerted by stopping the power steering function in a case where it is detected that the rotating speed of the motor is equal to or less than a predetermined value, for example, when a system is started.
Generally, when the voltage of a battery which is a power source of a power steering apparatus decreases, an electric current which flows into the motor decreases due to wiring resistance, the internal resistance of a circuit, or the like. Since the output torque of the motor is proportional to the electric current which flows into the motor, the output torque of the motor is reduced due to a decrease in the voltage of the battery, and the rotating speed of the oil pump decreases.
According to the conventional power steering apparatus disclosed in Japanese Patent No. 3652142, the power steering function is stopped regardless of the voltage of the power source to be applied to the motor in a case where the rotating speed of the motor is equal to or less than a predetermined value. Thus, it was necessary to take measures such that the power steering apparatus is not allowed to operate under the condition that a decrease in the power source voltage of the power steering apparatus is apt to occur, for example, when an engine is started.